This invention related to a new and novel CRT device for generating a bright color light spot of the type that is particularly useful for projection color television receivers and for information display.
During operation of cathode-ray tube devices which are used for generating a bright color light spot particularly when employed for projection color television, a large amount of heat is generated. This heat is generated at the face plate of the cathode ray tube, the temperature of the center of the face plate reaching 120.degree. C. or higher and the temperature of the phosphor coating of the face plate reaching even higher temperatures. As a result after a short period of operation the phosphor coating is damaged and the cathode ray tube can no longer be employed.
In order to prevent heating of the face plate it is frequent practice to provide a coolant between the outer surface of the face plate and an element such as a lens, provided adjacent to the outer surface of the face plate, for focusing images provided by cathode ray tube.
Masegawa U.S. Pat. No. 4,725,755, col. 20 shows use of a mixture of ethylene glycol and 20% to about 40% by weight of glycerol as a coolant provided between the lens and the face plate of the cathode-ray tube.
However, the index of refraction of this coolant (1.444) is insufficiently high as compared to that of the lens which is formed of polymethyl methacrylate (PMMA) face plate of the cathode ray tube. Further this coolant tends to turn yellow after a period of use.
Fukuda et al U.S. Pat. No. 4,824,224 shows the use of polyethylene glycol -200- as such a coolant.
Polyethylene glycol has the advantage of having a higher refractive index (1.459) and thus it tends to avoid formation of a "halo" effect due to a reduced difference in the indexes of refraction between this coolant and the lens. However polyethylene glycol has the disadvantage of insufficiently wetting the polymethyl methacrylate lens.